The present invention relates to a Raman laser apparatus that converts the wavelength of incident laser light. More particularly, the present invention relates to a low-temperature Raman laser apparatus which employs a simple cooling system and enables Raman conversion to be stably and efficiently effected at reduced cost by an easy operation.
Parahydrogen can take only an even rotational quantum number. It is a known technique as a parahydrogen Raman laser apparatus to convert incident light of frequency .nu..sub.p into light of frequency .nu..sub.p .+-..nu..sub.R by Raman conversion using the energy difference between the rotational levels J=0 and J=2 of parahydrogen (where .nu..sub.R is Stokes shift of parahydrogen; it is approximately 300 cm.sup.-1 to 400 cm.sup.-1). When carbon dioxide laser light is used as incident light, the wavelength of light frequency-converted by such a parahydrogen Raman laser apparatus is about 16 .mu.m, which is coincident with the absorption wavelength of uranium fluoride. Therefore, the laser system can be used as an infrared laser apparatus for molecular laser uranium enrichment. Further, the wavelength of the light frequency-converted by the parahydrogen Raman laser apparatus is also coincident with the absorption wavelengths of fluorides of other heavy metals. Accordingly, the laser system can also be used as an infrared laser for isotope separation of a heavy metal.
Incidentally, in the conventional parahydrogen Raman laser apparatus, parahydrogen gas, which is used as a medium, has heretofore been cooled by either of the following two methods: one in which parahydrogen gas is cooled to about 100.degree. K. by using liquid nitrogen (Appl. Opt. 19(1980)301; Appl. Phys. 57(1985)1504), and the other in which the medium is cooled to about 300.degree. K. under room temperature conditions (Appl. Phys. Lett. 47(1985)1033).
These conventional methods suffer, however, from the following problems. If the Raman laser medium is cooled by using liquid nitrogen, the plane wave Raman gain coefficient increases, which is effective for Raman conversion. However, the temperature conditions are severe upon the equipment. Accordingly, the mirror curvature radius and the distance between the mirrors change to a large extent, and a local change in the density of the medium causes a change in the optical path. In addition, since the difference between the medium temperature and the room temperature is large, the effect of heat externally applied to the cell on the temperature of the medium in the cell is large. In consequence of these, Raman conversion becomes unstable. Further, since liquid nitrogen is consumed at a high rate, the running cost is high. In the case of lowering the medium temperature under room temperature conditions, Raman conversion takes place stably. However, since the plane wave Raman gain coefficient is small, it is necessary to increase pump light power and also increase the propagation distance of light in the medium. As a result, damage to the optical parts is invited, and it is difficult to achieve a high conversion efficiency.